Thus a change is effected in the properties of water by heating it ever
so little. If it is more strongly heated a still greater change takes
place. You know what happens when a saucepan containing water is put on
the fire. The water gets hotter and hotter, then it begins to simmer,
and finally, when it reaches 212°, it boils away into steam, which
passes into the air and disappears. If the boiling is carried on long
enough all the water vanishes. It looks at first as if the water had
been destroyed by the heat. In reality, however, not a particle of water
has been destroyed. It has merely changed its state. The heat has
altered it from the state of liquid water into that of gaseous water,
=vapour= or =steam=.
Try the same experiment with a tea-kettle instead of a saucepan, but
only put a little water in the tea-kettle, and shut the lid well down.
Then, as soon as the water begins to boil, the steam will shoot out of
the spout in a jet; and this will go on as long as any water remains in
the kettle.
The steam, as it comes out of the spout, is so hot that it will scald
you if you put your finger in it. But you may satisfy yourself that it
is very hot, without scalding your fingers, by holding a stick of
sealing-wax in it. The wax will soften, just as if you held it before
the fire. Moreover, if you look through the steam, just where it leaves
the spout, you will see that it is quite transparent; it is only at some
little distance from the spout that it loses its transparency, changes
into a white opaque cloud, and rapidly vanishes in the air.
33. =The taking away of Heat from Steam causes the steam to change into
Hot Water.=
Now take a cold spoon, or a cold plate, and hold it against the jet of
steam, for a moment or two. When you take it away, you will find that it
is quite wet, being covered with drops of warm water, and, moreover, the
cold spoon, or plate, has become warm. And if you fit a long cold metal
pipe to the nozzle of the tea-kettle, you will find that no steam at all
issues from the end of the pipe, but only water, while the pipe becomes
warmed.
Thus the heat passes from the fire into the saucepan, or kettle, and
thence to the water which they contain; the water gets hotter and
hotter, and, when it has taken in a certain quantity of heat, it becomes
steam, or vapour of water. When the steam comes against the cold plate,
or passes through the cold pipe, it gives up the heat it has taken in to
the plate, or the metal of the pipe. They carry off the heat which kept
the water in a condition of a =vapour=, and so it passes back into the
condition of =liquid=.
Thus steam and water are two conditions of the same thing, =water=; they
are effects of the quantity of heat which the water has taken in.
34. =When Water is changed into Steam, its Volume becomes about 1,700
times greater that it was at first.=
If you could measure and weigh the water in your kettle to begin with,
and then measure and weigh all the steam into which the heat of the fire
changes it, you would find that the bulk of the steam was nearly 1,700
times as great as the bulk of the water, though the weight of the steam
would be exactly the same as that of the water. If you had a small
square cup like a die, the inside measure of which was exactly one inch
each way, it would hold one cubic inch of water. If this cup full of
water were heated till all the water was turned into steam, the steam
would nearly occupy a cubic foot; since there are 1,728 cubic inches in
a cubic foot. A cubic inch of water weighs 252½ grains, and the steam
into which it is converted has just the same weight. Thus we may say
that steam is water expanded by heat into a vapour which is of 1,700
times less specific gravity than water. On the other hand, a pint of
steam allowed to cool, becomes converted into a quantity of water, which
measures only 1/1700th of a pint, though it weighs just as much as the
whole pint of steam did. The steam, therefore, is =condensed= to a
1/1700th of its volume of water.
The power with which water expands when it is converted into steam is
very great. If you were to stop up the nozzle of the tea-kettle, the
steam, inside the kettle, in trying to expand, would burst open the lid;
and if you were to fasten down the lid, it would pretty soon burst the
kettle itself. You sometimes hear of the strong boilers of steam-engines
being burst in this way.